Electroplating is a well established procedure for applying a silver layer over a conductive surface. However, despite the well established nature of the general procedure, it is always a challenge to consistently apply a uniform, smooth, chemically pure silver layer. Even with the exercise of care, consistently reproducible results are not always obtained.
Various techniques have been attempted in the prior art to improve the quality and reproducibility of plated silver layers. Examples of measures utilized in the prior art include periodically reversing the polarity of the plating current during the plating process (U.S. Pat. No. 2,451,341); periodic current reversal combined with agitation of the plating bath (U.S. Pat. No. 2,678,909); plating and then deplating the substrate with a material such as copper from a copper-containing bath prior to transferring the substrate to a plating bath containing the metal ultimately desired to be plated on the substrate (U.S. Pat. No. 3,661,728); anodic cleaning and application of intervening strike layers of silver or other metals with simultaneous plating of a dummy article (U.S. Pat. No. 3,878,065); successive treatment in an anodic alkali hydroxide bath, an acid soaking bath, cathodic activation in a copper bath, and water rinsing prior to silver plating (U.S. Pat. No. 4,069,110); and alternating cathodic and anodic treatment in a neutral electrolyte solution (U.S. Pat. No. 4,127,450).
Despite the development of the art, there has remained a continuing need for improved techniques of applying a plated silver layer onto a conductive surface. The problem is particularly acute in the application of silver layers for silver thermocompression bonding of composite materials such as multi-layer printed circuit boards because the quality and reproducibility of the silver layers have a critical effect on the quality of the bonds produced between layers.